The Mil Mi-8 is a medium twin-turbine helicopter, originally designed by the Soviet Union, and now produced by Russia. In addition to its most common role as a transport helicopter, the Mi-8 is also used as an airborne command post, armed gunship, and reconnaissance platform. Along with the related, more powerful Mil Mi-17, the Mi-8 is among the world's most-produced helicopters, used by over 50 countries. As of 2015, it is the third most common operational military aircraft in the world.

Mikhail Mil originally approached the Soviet government with a proposal to design an all-new two-engined turbine helicopter after the success of the Mil Mi-4 and the emergence and effectiveness of turbines used in the Mil Mi-6; but the military argued against a new helicopter, as they were content with the current Mil Mi-4. To counter this, Mikhail Mil proposed that the new helicopter was more of an update to new turbine engines rather than an entirely new helicopter; this persuaded the council of ministers to proceed with production. Due to the position of the engine, this enabled Mikhail Mil to justify redesigning the entire front half of the aircraft around the single engine (designed by Oleksandr Ivchenko at OKB-478 in Zaporizhzhia, Ukraine, originally for fixed wing aircraft as all other soviet turbines had been up to that point).

The prototype, which was named V-8, was designed in 1958 and based on the Mil Mi-4 with a larger cabin. Powered by an AI-24 2,010 kW (2,700 shp) Soloviev turboshaft engine, the single engined V-8 prototype had its maiden flight in June 1961 and was first shown on Soviet Aviation Day parade (Tushino Air Parade) in July 1961.

During an official visit to the United States in September 1959, Nikita Khrushchev took a flight in the S-58 presidential helicopter for the first time and was reportedly extremely impressed. On Khrushchev's return, he ordered the creation of a similar helicopter, which was to be ready for the return visit by the American president, to save face. A luxury version of the Mi-4 was quickly created and Khrushchev took an inspection flight, during which Mikhail Mil proposed that his helicopter in development was more suitable. However, it would be necessary to have a second engine for reliability. This gave Mikhail Mil the power under the orders of Khrushchev to build the original two-engined helicopter, which for the first time in Soviet history would need purpose-built turbine engines, rather than those adapted from fixed wing aircraft (as in the Mil Mi-6 and the first prototype V-8) and an entirely new main rotor gear box that would be designed in-house for the first time. In May 1960, the order was given for Mikhail Mil to create his twin engine helicopter. The Sergei Isotov Design Bureau accepted the task of creating the engines.

The second prototype (still equipped with the one turbine engine as the Isotov engines were still under development) flew in September 1961.

Two months after the engines were completed by Isotov, the third prototype designated V-8A equipped with two 1,120 kW (1,500 shp) Isotov TV2 engines, made its first flight piloted by Nikolai Ilyushin on 2 August 1962, marking the first flight of any Soviet helicopter to fly with purpose built gas turbine engines. The aircraft completed its factory based testing in February 1963.

The fourth prototype was designed as a VIP transport, with the rotor changed from four blades to five blades in 1963 to reduce vibration, the cockpit doors replaced by blister perspex slides and a sliding door added to the cabin.

The fifth and final prototype was a mass production prototype for the passenger market. In November 1964, all joint testing had been completed and the soviet government began mass production. Production started in the Kazan Production Plant, with the first aircraft completed by the end of 1965.

The Soviet military originally showed little interest in the Mi-8 until the Bell UH-1's involvement in the Vietnam war became widely publicised as a great asset to the United States, allowing troops to move swiftly in and out of a battlefield and throughout the country. It was only then that the Soviet military rushed a troop-carrying variant of the Mil Mi-8 into production. By 1967, it had been introduced into the Soviet Air Force as the Mi-8.

There are numerous variants, including the Mi-8T, which, in addition to carrying 24 troops, is armed with rockets and anti-tank guided missiles. The Mil Mi-17 export version is employed by around 20 countries; its equivalent in Russian service in the Mi-8M series. The only visible difference between the Mi-8 and Mi-17 is that the tail rotor is on the starboard side (right side) of the Mi-8, whereas in Mi-17 it is on the port side. Also Mi-17 also has some improved armour plating for its crew. The naval Mil Mi-14 version is also derived from the Mi-8. The Mi-8 is constantly improving and the newest version still remains in production in 2016.

SOURCE: Wikipedia